Conventional TPEs have not reached the level of vulcanized rubber from a viewpoint of quality in the field of a use or uses shared by the vulcanized rubber among the extensive fields of applications as rubbers, and has been limited in their application to the field shared by vulcanized rubber.
In particular, at present, no TPEs have been available that are comparable to having the desired compression set of the vulcanized rubber.
Typical of these TPEs include a styrene/butadiene block polymer or a styrene/isoprene block polymer.
In styrenic TPEs, it has been impossible to obtain those having desired compression set unless the molecular weight of these TPEs are higher than a certain level, and, moreover, the TPEs of these types are very poor fluidity properties that can be achieved only by sacrificing the moldability which is a greatest advantage the TPEs have. Still further, the effect in the improvement tried for the compression set has been insufficient as compared with that of the vulcanized rubber.
Polyphenylene ethers, used in the present invention, are resins used as engineering plastics in a wide field of industries. As is well known in the present industrial field, the polyphenylene ethers can be used in combination with polystyrene to vary the thermal resistance of the polystyrene depending on the mixing proportion thereof.
With regard to the combination of styrenic block copolymers and polyethylene ethers, U.K. Patent Publication No. 1,275,141, for example, discloses that a combination of a polyphenylene ether with a polybutadiene-containing random or block copolymer (e.g. an A-B-A type block copolymer) optionally containing a styrenic copolymer brings about desired impact resistance and processability. This art teaches that a styrenic block copolymer is effective for improvement in the impact resistance of a styrenic resin combined with a polyphenylene ether resin.
U.S. Pat. No. 3,639,508 (or U.K. Patent Publication No. 1,274,050) discloses that a composition comprising 1 to 45% by weight of a polyphenylene ether and 99 to 55% by weight of a styrenic block copolymer (e.g. an A-B-A type block copolymer) shows improved heat strain resistance. In this connection, the heat strain resistance herein mentioned is a value obtained by measuring the temperature at which a sample hung under a fixed load (100 psi) with temperature increasing at the rate of 2.5.degree. C./min has elongated by 10%. This measured value resembles the value according to the HDT test method which evaluates the thermal resistance of resins, and is a value for physical properties which is basically different from the compression set to be evaluated in the present invention.
International Publication No. WO 81/02020 also discloses that a composition comprising a hydrogenated A-B-A or unsaturated A-B-A block copolymer, a polyphenylene ether and a small amount of plasticizer selected from a phthalate, an alkyladipate and a paraffin type oil has good processability and impact resistance. This art also teaches that additon of a hydrogenated A-B-A type block copolymer or a non-hydrogenated A-B-A type block copolymer and a plasticizer is effective for improvement in the processability and the impact resistance of a polyphenylene ether resin, and is not directed to an invention aiming to improve a thermoplastic elastomer as in the present invention.
The present invention is essentially different from the inventions disclosed in the patents as described above as prior art. The composition obtained according to the present invention is a rubber-like soft material showing excellent compression set, and is a thermoplastic elastomer requiring no vulcanizing step and yet having the same moldability as ordinary thermoplastic resins possess.
The present inventors have made extensive studies on vinyl aromatic hydrocarbon type elastomer compositions having excellent compression set and moldability, and have discovered that a composition comprising (i) radial type or straight chain type block copolymers of which number average molecular weight and content of vinyl aromatic hydrocarbon are in a specified range [components (a) and (b)], (ii) a polyphenylene ether resin [component (c)] and (iii) a non-aromatic rubber softening oil [component (d)] has both the desired compression set and good fluidity. The present invention has been thus accomplished.